Studies are being carried out on installation of a radio communication base station apparatus using a narrow cell (femtocell) as its coverage having a cell radius of several tens of meters or less in a general household or building like office (referred to as “femtocell base station,” or “Home Node-B,” hereinafter referred to as “femtocell base station” or “HNB”). FIG. 1 shows a radio system configuration when a femtocell base station is installed. Portable terminal 102 (hereinafter referred to as “femtocell connected terminal” or “HUE”) located in a building where femtocell base station 101 is installed performs data transmission with femtocell base station 101 via a radio channel. Femtocell base station 101 uses fixed network 103 (e.g., xDSL, FTTH) such as an optical fiber introduced into each household as a backbone and is connected to core network 105 via Internet 104 or the like. Installation information of femtocell base station 101 is managed by network management apparatus 106 in the core network.
Compared to a conventionally used wide-area cell (that is, macro cell) having a cell radius of on the order of 0.5 to 1.5 kilometers, the femtocell has the following technological features.                (1) A femtocell base station has low transmission power and a narrow cell radius.        (2) Not many terminals are simultaneously connectable to a femtocell base station.To be more specific, while several tens of terminals or more are simultaneously connectable to a macro cell, several terminals are simultaneously connectable to a femtocell.        (3) It is difficult for a communication carrier to design the layout of a femtocell in advance.        (4) A user's fixed network is incorporated in a network configuration. In the macro cell, terminal 108 is connected to core network 105 via base station 107 and radio access network 109 managed by a communication carrier.        
When femtocell base stations are widely spread in an urban area, it is anticipated that it will be difficult to secure femtocell-specific frequency bands because existing cellular systems use all operating frequency bands. Therefore, in the urban area, a macro cell and femtocell are assumed to be hierarchically arranged (that is, hierarchic cell arrangement). That is, the macro cell and femtocell are assumed to share the same frequency band in the same region. Furthermore, only registered users (CSG: Closed Subscriber Group) may be authorized to make connections to femtocell base stations for the purpose of preferentially providing a communication service to users who install the femtocell base stations.
In such an operation of a hierarchic cell arrangement of a macro cell and femtocell, interference of uplink radio channels between the macro cell and femtocell becomes problematic. That is, a throughput increase in one cell may lead to an increase in interference/throughput decrease in the other, that is, occurrence of trade-off.
One example of radio interference between the macro cell and femtocell can be radio interference on an uplink that a macro cell connected terminal provides to a femtocell base station. Especially, when propagation loss between the macro cell base station and macro cell connected terminal grows, the radio transmission power of the macro cell connected terminal increases, and therefore the problem of interference provided from the macro cell connected terminal to the femtocell base station becomes serious. For this reason, when a CSG femtocell base station is installed, countermeasures need to be taken against interference with the femtocell base station from the macro cell connected terminal that cannot be connected to the femtocell base station.
Conventionally, when performing uplink multicarrier transmission such as OFDMA or SC-FDMA, a method is disclosed whereby different frequency bands are assigned to a plurality of cells in assignment of resource blocks used for uplink data transmission to thereby reduce interference between cells (e.g., see Patent Literature 1). As described in Patent Literature 1, uplink interference information is exchanged between base stations or between a base station and network management apparatus, and frequency band assignment so as to reduce uplink interference is determined.
FIG. 2 shows a sequence diagram of an interference reduction method described in Patent Literature 1. In FIG. 2, base stations 203 (Node-B1) and 204 (Node-B2) located close to each other measure the amount of interference of an uplink radio channel (ST201) and report the measured amount of interference to network management apparatus 205 (NW Node) (ST202). Furthermore, terminals 201 (UE1) and 202 (UE2) connected to base stations 203 and 204 respectively measure the reception levels of respective downlinks (ST203) and report the measurement results to network management apparatus 205 via the base stations (ST204). Network management apparatus 205 assigns uplink frequency bands used in the respective cells so that interference between the cells is reduced most based on the information reported from the base stations or terminals (ST205). Network management apparatus 205 then reports the frequency band assignment (that is, operating bands) to base stations 203 and 204 (ST206). Base stations 203 and 204 perform uplink scheduling according to the reports (ST207) and assign uplink operating frequency bands to the terminals (ST208).
Here, according to the method shown in FIG. 2, network management apparatus 205 adjusts the operating frequency bands based on interference information of all cells. On the other hand, as another method, information on the amount of interference can also be directly exchanged between base stations 203 and 204 as shown in ST209 of FIG. 3. For example, the 3GPP (3rd Generation Partnership Project) LTE (Long term evolution) standard defines a signal for directly exchanging information on the amount of uplink interference per band (UL interference control signaling (OI, HII) report) between base stations (see Non-Patent Literature 1). To be more specific, “UL Interference Overload Indication” is defined as the signal for reporting the amount of uplink interference per band. Furthermore, “UL High Interference Indication” is defined as a signal for warning neighboring cells that the own cell will use a specific frequency band. The base station that receives these signals performs frequency band assignment different from that of the neighboring cells (ST210) and transmits information on the frequency band assignment to the terminal (ST211). The amount of uplink interference is thereby reduced.